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Signals of a warming
planet can be read in the behavior of a common little bird. Only five
inches long, the tree swallow (Tachycineta bicolor) is a migratory
songbird with iridescent blue-green wings and a gleaming white belly.
This small creature is a highly sensitive climate-change indicator across
a band of temperate North America, from New England to the Pacific Northwest.
By studying the tree swallow’s nesting patterns, researchers have
discovered that spring warming now arrives more than a week earlier in
places like Massachusetts and Wisconsin because of global climate change.

Tree swallows, agile
and swift, winter in places like coastal South Carolina, roosting in huge
numbers in wax-myrtle trees and wheeling across the sky in pursuit of
flying insects.

Then in early April
they migrate north to claim and defend nesting sites in tree cavities
and bird boxes. April truly is the cruelest month for these birds. Tree
swallows must survive the last freezing weeks of northern winter until
flying insects emerge during the first or second week of May. “They
have some really lean times,” says Cornell University ecologist
and evolutionary biologist David W. Winkler.

The birds fatten
up when insects start flying again. But tree swallows are patient family
budgeters. Only when they can catch enough insects do they start laying
eggs. Tree swallows time their breeding to the moment when insect populations
explode in abundance, occurring during the first vigorous spurt of northern
warming. Thus, if you study when these birds lay eggs, you’ll know
when spring arrives across the forested regions of the continent’s
northern tier.

Over a period of
four decades, volunteer bird watchers tracked about 21,000 tree-swallow
nests in the United States and Canada, sending data cards to Cornell University’s
Laboratory of Ornithology. With this information, Winkler and colleagues
learned that tree swallows were laying eggs nine days earlier on average
in 1991 than they had in 1959. The birds, Winkler says, are responding
to global warming and earlier spring temperatures. This study, however,
does not address nesting data throughout the 1990s, the warmest decade
in recorded history.

Dozens of bird species
with various life histories are similarly nesting earlier than before.
And “a lot of bird species—from short-term to long-term migrants—are
going north one to three weeks earlier than they did 20 years ago,”
says Jeff T. Price, director of Climate Change Impact Studies at the American
Bird Conservancy, based in Boulder, Colorado. “Very few species
are migrating later.”

Plants are flowering
sooner than they did a few decades ago in many regions. Some mammals are
breaking hibernation earlier. Many species have shifted their ranges toward
the poles.

The Earth has warmed
by 1.1 degrees Fahrenheit since 1900, an unprecedented temperature rise
over the past millennium. Two-thirds of this warming has occurred just
within the past 30 years, including an abrupt temperature spike upward
starting in the mid-1990s.

Over the past several
decades, hundreds of research projects have documented wildlife species’
responses to global warming. Working independently, two research teams
analyzed many of these previously published papers, offering a startling
picture of a changing climate’s impacts on wild creatures.

Terry L. Root, a
Stanford University ecologist, and colleagues used statistical tools to
analyze 143 separate research studies involving a total of 1,473 species
of plants and animals. Each original study examined a correlation between
climate change and a biological response in plants or animals somewhere
in the world.

More than 80 percent
of species had made a temperature-related shift in response to climate
change, Root and co-authors discovered. Spring events—migrating,
blooming, and nesting—had shifted earlier by an average of five
days per decade over the past 30 years for temperate-zone species.

In another study,
Camille Parmesan, a biologist at the University of Texas at Austin, collaborated
with Wesleyan University economist Gary Yohe to analyze studies of 172
species of plants, birds, butterflies, and amphibians. Spring events had
advanced by a mean of 2.3 days per decade over a time period of 16 to
132 years, with a median of 45 years.

Terrestrial ecosystems
have experienced greater warming than deep oceans. But marine creatures
are moving poleward too, particularly in intertidal areas and along continental
shelves; shallow water is affected more by temperature than deeper ocean
water. Around California’s Monterey Bay, warmer water has changed
community structure, driving many invertebrates northward and out of the
bay, and allowing other species to replace them from the south.

In the North Atlantic
Ocean, five of eight commercially important groundfish species have moved
north in response to warming. Growing numbers of fish species common to
Caribbean waters have been found at a reef off Beaufort, North Carolina,
over the past 25 years. “The reef population here is much more tropical
than it was,” says R.O. Parker, a research fishery biologist with
the NOAA National Marine Fisheries Service in Beaufort.

Just as wild creatures
respond to a rapidly changing climate, so do row crops, forest plantations,
and disease-carrying insects. Global warming is likely to affect many
ecosystems on which we depend.

The Earth’s
average surface temperature will continue growing hotter over the next
half-century at least. Natural variation has a part in this change. But
human actions, scientists say, are mostly to blame. We are burning so
much carbon once stored in the Earth—oil, natural gas, and coal—that
we are altering the world’s climate.

Since the industrial
revolution began 200 years ago in Europe, people have burned fossil fuels,
increasing the amount of carbon dioxide and other so-called “greenhouse”
gases sent into the Earth’s atmosphere.

The “greenhouse
effect,” of course, is a natural phenomenon. Greenhouse gases—carbon
dioxide, methane, and nitrous oxide—naturally reflect a portion
of the sun’s solar radiation back to Earth and warm the planet.
This entrapment sustains the global surface temperature. Without the greenhouse
effect, the planet would be a ball of ice.

The problem is that
our fossil-fuel burning increases the greenhouse effect, trapping more
of the sun’s energy and causing the Earth’s temperature to
rise.

“When we use
the atmosphere as a sewer, something happens,” says Stephen H. Schneider,
a professor of geosciences at Stanford University.

In 1988, the nations
of the world appointed the Intergovernmental Panel on Climate Change (IPCC),
comprising more than 2,000 leading experts to assess the science and economics
of climate change. In 2001, the IPCC published its third major report,
forecasting that the Earth could warm by 2.5 to 10.4 degrees Fahrenheit
by 2100.

Global climate change
generally affects higher latitudes (colder places closer to the poles)
more than subtropical and tropical regions. In Alaska, for example, average
temperatures have risen by about 3.6 degrees Fahrenheit over the past
30 years.

“There is not
uniform warming” around the world, says James White, a climate scientist
at the University of Colorado. “Europe and Asia are warming up quite
a bit faster and quite a bit more (in degree) than North America.”
Indeed, the entire U.S. Southeast has actually cooled very slightly over
the past century, though South Carolina has warmed by about 1 degree Fahrenheit.

The Arctic and Antarctic
have experienced the most abrupt climate change. Since the 1950s, higher
polar temperatures have diminished the extent of Arctic sea ice by 30
percent in spring and summer. Now floating masses of sea ice are splitting
apart. The massive Greenland ice cap melted more last summer than in recorded
history.

Unless nations support
swift, substantial cuts in greenhouse-gas emissions, Arctic sea ice in
summer time could diminish 60 percent by 2050, according to a 2001 assessment
by the United Nations Environment Programme. Melting of polar ice and
permafrost pushes more freshwater into the Arctic Ocean, which could stimulate
explosive shifts in ocean circulation and climate around the world.

How high could temperatures
go, and how quickly?

“That increasing
greenhouse gas concentrations contributed to (global) warming is not in
serious dispute,” notes a 2001 report by the George C. Marshall
Institute, a nonprofit organization known for its criticism of mainstream
climate-change research.

Skeptics argue that
global warming is not cause for alarm. More warming has already brought
some positive effects, they say. Since the 1960s, growing seasons have
lengthened by one to four days per decade in the Northern Hemisphere,
particularly in higher latitudes. Some cold-weather places now have more
warm days to grow crops, and people have lower heating bills in winter.
Most people in industrial nations, skeptics say, would simply adjust to
a changing climate.

Patrick Michaels,
a Virginia state climatologist and well-known global-warming contrarian,
has argued that the planet will likely heat up by 2.8 degrees Fahrenheit
by the year 2100. His forecast falls within the lower range of the IPCC
assessment, yet it is almost triple the amount of last century’s
warming.

But even modest global
warming could spell doom for creatures in certain vulnerable habitats.
Within 50 years, some small oceanic islands could be drowned by large
storms and rising sea levels. By 2050, coastal wetlands and mangroves
in some regions could be permanently submerged. Many species in northern
intertidal zones will face localized extinctions because of heat stress.
Many coral reefs worldwide probably will be gone, killed by warming water
and other impacts.

“Some ecosystems
will find it virtually impossible to adapt to a fair amount of climate
change, no matter how slowly it arrives,” says Thomas E. Lovejoy,
president of the Heinz Center for Science, Economics, and the Environment,
based in Washington, D.C.

Mountaintop glaciers
in warm-weather countries almost certainly will melt rapidly. High-altitude
ice and snow in places like Tanzania have already receded so quickly that
they will be gone in just 20 more years, says Lovejoy.

Go see them while
you can: the famous snows of Kilimanjaro will soon be no more.

HISTORIC
EVIDENCE AND MODELS

How do scientists really know
that the Earth’s surface is warming?

Beginning in the mid-1850s,
governments established a network of instruments that have measured surface
temperatures across land and marine regions of the Northern Hemisphere.
As a result, researchers have continuous documentation of the hemisphere’s
climate over the past 140 years.

Two-thirds of recorded global
surface warming has occurred since the end of the Vietnam War, and the
decade of 1990s included nine of the 10 hottest years since the Civil
War began. The warmest year was 1998; the third and fourth hottest were
1990 and 1995. The twenty-first century got off to a blazing start in
2001 and 2002; the latter was the second warmest year on record.

“We are reaching a fundamentally
different level of temperature change on the planet,” says White.

The Earth’s biological
and physical “archives” also contain clues to ancient temperatures.
Bubbles of air trapped in Antarctic ice show that for the past 400,000
years carbon-dioxide levels in the atmosphere have closely followed global
temperatures as revealed in ice cores. It seems clear that carbon-dioxide
levels and temperature generally move in long-term tandem.

By studying tree rings and
ice cores, researchers have concluded that recent surface temperatures
are probably higher than any comparable era since the Middle Ages. The
period from 1970 to 2000 was likely the Northern Hemisphere’s warmest
three-decade period of the past thousand years, according to the Climatic
Research Unit at the University of East Anglia in Great Britain.

Global average temperature
will continue to rise because of heat-trapping pollution, climate scientists
agree. Cars, factories, and power plants emit about 24 billion tons of
carbon dioxide a year. Forests and the ocean, which help soak up heat-trapping
gases, cannot absorb swiftly rising emissions levels. Instead, the atmosphere
will have to take up more of the greenhouse-gas load.

Pollution from cars
and factories does not affect climate immediately. It takes 10 to 20 years
for today’s pollution to begin showing in the temperature record.
But some greenhouse gases have long lives. A century from now, our descendants
will likely experience rising temperatures caused by current pollution.

WHAT’S
THE CAUSE?

Could the recent spike in global
surface temperature and Arctic meltdown be attributed to a natural, cyclical
phenomenon?

Numerous forces affect global
climate change, including solar variability that nudges Earth’s
temperatures up or down, volcanic explosions that spew dust into
the air and cool the planet, and greenhouses gases that warm the Earth.

But only one phenomenon, most
climate scientists say, can satisfactorily explain the sudden global temperature
jump at the end of the twentieth century. “The recent warming, particularly
over the past 20 years, cannot be attributed to anything but human involvement
in greenhouse gases,” says White.

Scientists use general-circulation
computer models to estimate how the climate will respond to increasing
greenhouse gases. These models—which Schneider calls “mathematical
expressions of physical laws that we believe in”—are extraordinarily
complex, with many built-in uncertainties.

Early models could not begin
to approximate the climate system’s intricacy. But newer models
incorporate more of the hydrological, geological, and biological factors
that influence climate: dust and soot, volcanoes, tree cover, ocean currents,
among many others.

Yet even the best models cannot
address factors that will dramatically affect the pace of future warming,
such as the behavior of water vapor, clouds, and atmospheric particles.
These “feedback” mechanisms could significantly increase warming
or damp it down.

The problem is that nearly
all feedback mechanisms in current models are positive, skeptics say.
Computer models show that as the Earth grows warmer, the atmosphere increasingly
traps energy from the sun. As a result, the Earth’s atmosphere becomes
moister, which exacerbates surface warming, which makes the atmosphere
even moister.

Nevertheless, the Earth’s
climate probably doesn’t work like that, says John Christy, an Alabama
state climatologist and an IPCC report lead author. As human society increases
greenhouse-gas emissions, larger-than-expected amounts of energy apparently
leak into space from the Earth’s surface, therefore slowing global
warming.

“There is increasing
evidence that energy escapes (from the Earth’s surface into space)
through clear, dry areas of the planet,” says Christy. “Those
areas become drier and larger as (atmospheric) carbon dioxide increases.”

This process, he says, is part
of the planet’s self-regulating mechanism, in which climate tends
to return to the long-term mean after temporarily becoming colder or warmer.
Christy also argues that greenhouse-gas emissions can be blamed for only
a small fraction of global warming over the past century.

But only a tiny minority of
knowledgeable scientists—just a handful in number—holds Christy’s
view that human society is not primarily responsible. The overwhelming
majority of climate scientists argue that humankind is largely to blame
for rising temperatures.

Even so, climate scientists
acknowledge their lack of an airtight case: they cannot prove beyond any
doubt that increasing greenhouse gases from human activities have warmed
the Earth and will continue to do so.

Researchers, however,
do have “multiple lines of evidence and fingerprints” that
the planet has warmed primarily because of greenhouse-gas emissions, Schneider
says. Climate scientists have gathered detailed circumstantial evidence
that human activities are mostly to blame for the rising average global
surface temperature.

Scientists are especially confident
that they understand the workings of carbon dioxide, the most influential
heat-trapping greenhouse gas. Before 1750, global carbon-dioxide load
in the atmosphere was 280 parts per million. Due to increased pollution,
the atmospheric load has risen to 370 parts per million, one-third above
pre-industrial levels.

The Earth’s
atmosphere, according to climate-computer models, is likely to contain
double the pre-industrial levels of heat-trapping gases within the next
50 years, causing a rise of two to four degrees Fahrenheit in global average
surface temperature. Unless the global community establishes policies
to control greenhouse gases, the atmospheric load could rise to 1,000
parts per million by the end of the next century, causing potentially
explosive climate changes.

But that assumption is wrong,
says Michaels. Instead, developing countries will soon bypass older, dirty
technologies and employ technologies that reduce heat-trapping emissions.
“Very clean technologies are being developed by affluent countries,”
says Michaels, “and these technologies are going to be sold to developing
countries.”

Still, an important question
remains: should governments wait to act until researchers have complete
scientific certainty before taking steps to control greenhouse-gas pollutants?
No, climate scientists say. By the time researchers have “smoking-gun”
evidence, it could be too late to prevent environmental, social, and economic
catastrophes in some regions.

BIOLOGICAL
IMPACT

Animals have always responded
to temperature fluctuations throughout their evolutionary history. Couldn’t
most creatures adjust to climate change? Isn’t that what natural
selection is all about?

Climate change hasn’t
harmed the tree swallow, which nests earlier without ill effects. The
tree swallow still thrives by eating flying insects. And it could probably
adapt to climate change over the next century. “Tree swallows should
do pretty well,” says Winkler, because flying insects would still
provide a consistent food source in a hotter era.

But other species might not
be so fortunate.

Plant and animal species thrive
only within certain temperature ranges. They must relocate to a new habitat
when a particular location gets too hot or too cold for them. When the
climate cools, species generally move in the direction of the Equator
and down mountainsides. When the climate warms, most species head toward
the North Pole in the Northern Hemisphere and the South Pole in the Southern
Hemisphere, or they move up mountainsides.

In the future, some species
could relocate to habitat many miles away, though their food sources might
not follow as far. Meaning that an animal species might relocate to a
different place than its prey does. “We could see a tearing apart
of natural communities,” separating animals from their food sources,
says Root.

Some animals with narrow foraging
requirements might not survive in a rapidly warming world. A bird, for
example, that relies exclusively on a particular species of insect or
worm could starve or fail to reproduce. The bird’s climate-induced
migration could be out of synchrony with its prey’s migration; predator
and prey could go to different places.

“Those species with
any kind of specialized relationship with prey organisms could really
get into trouble,” says Winkler.

Moreover, cities, farms, ranches,
plantation forests, and other intensively managed landscapes could block
many climate-induced wildlife migrations. Species could get squeezed between
rising temperatures behind them and development in front of them. Thus,
rapid climate change could cause numerous extinctions.

Higher temperatures would likely
spawn faster rates of evaporation and precipitation, so that some regions
would have shorter and more intense rainy seasons. Other regions would
have longer droughts, endangering some crops and causing a drop in global
food production, though most U.S. agricultural sectors could adapt to
a warmer climate. Longer droughts would also diminish water supplies in
many areas, exacerbating existing shortages.

Mosquitoes and many other disease
carriers will probably increase their range due to warmer and wetter weather.
Illnesses such as malaria and dengue fever will probably become even more
prevalent in some developing nations. “A warmer world would be a
sicker world,” says Andrew P. Dobson, an epidemiologist and ecologist
at Princeton University.

PLANETARY
MONKEYING

Some experts had hoped that
the Kyoto Protocol of 1997 would be the answer to global climate change.
It is an international treaty that seeks to control greenhouse-gas emissions
to below 1990 levels by 2012, applying mandatory standards on advanced
industrial nations that ratified it. The problem is that the Kyoto’s
provisions would only amount to a two to five percent cut in greenhouse-gas
emissions. “Kyoto really does little to materially affect the climate,”
says Roger A. Pielke, Jr., a University of Colorado political scientist.
The Kyoto treaty is far too little, too late, he says.

President George W. Bush announced
in 2001 that the United States would not ratify the Kyoto treaty two years
after the U.S. Senate declared it dead. The U.S. government saw Kyoto
as unfeasible, because most of its costs would have fallen on the shoulders
of American companies. Instead, the Bush Administration has sought voluntary
commitments from companies to reduce emissions, arguing that the science
addressing climate change is uncertain and that human factors are not
clearly contributing to global warming.

This stance sparked international
criticism from U.S. economic partners that point out that the United States
produces about 25 percent of the world’s carbon-dioxide emissions.
Japan and Australia together produce about 6 percent; all of Western Europe
produces 16 percent.

Many traditional U.S. allies
have argued for strong steps to control greenhouse gases. Prime Minister
Tony Blair of Great Britain recently laid out ambitious plans to cut greenhouse
emissions by 60 percent in the next five decades.

Senators John McCain (R-Arizona)
and Joseph I. Lieberman (D-Connecticut) have joined in proposing legislation
that would require mandatory though small reductions in greenhouse emissions
by 2010 and sharper ones by 2016.

More than any other nation,
the United States has the potential to disrupt the world’s climate.
Yet this country could adapt better than most. Americans have the scientific
skills, financial resources, technology, and social and political structures
to adjust to all but the most extreme scenarios over the next century.

American agriculture,
for example, probably would not be harmed overall by climate change; in
fact, warming could even help farmers in some regions. In the Southeast
U.S., however, climate change could spell trouble for many farmers and
foresters. Agriculture is already a marginal activity in this region,
and farmers would face increased water shortages for irrigation, potentially
driving them out of business. Forests would likely endure greater numbers
of destructive pests and slower growth because heat stress could weaken
trees, says Milt Brown, South Carolina state climatologist.

As sea levels rise, U.S. coastal
cities could retreat or build seawalls, though that would be expensive.
Public-health resources could be mobilized to fight emerging and re-emerging
diseases spawned by a warmer climate.

Poor people in some developing
nations, however, could face calamities if the climate continues to warm
rapidly. “What’s not dangerous in the U.S. might be dangerous
somewhere else,” says Tom Wigley, a senior scientist at the National
Center for Atmospheric Research, based in Boulder, Colorado. Residents
of flood- or drought-prone regions—such as coastal Bangladesh, the
African desert Sahel region, and small oceanic islands—would likely
have to relocate. “In some cases . . . we’ve probably gone
beyond the state of adaptation (to global climate change),” Rajendra
Pachauri, chairman of the IPCC, has said.

Now the Earth’s climate
system is undergoing changes exacerbated by pollution, experts say. Nevertheless,
many Americans believe that we could not disrupt something as vast as
the Earth’s climate, and that natural variability must be solely
to blame.

“The average person,”
says White, “does not have a good grasp of just how important human
beings are—not only in the climate system but in all phases of the
planet’s systems. There’s nothing on this planet that people
don’t monkey with.”

_________Sidebars:

Momentum
change

Since the early 1990s, the
momentum has shifted in scientific debates about global climate change.
A decade ago, many researchers argued that global warming could be attributed
exclusively to natural variation. Some doubted that the planet was warming
at all. But the Earth’s surface is undoubtedly growing hotter, especially
near the poles. Moreover, a growing body of circumstantial evidence indicates
that humans have a strong hand in this change.

Experts are now more certain
in answering one fundamental question over the past 12 years: are people
contributing to global warming?

In 1990, the First Assessment
Report of the Intergovernmental Panel of Climate Change (IPCC) reported:
“The unequivocal detection of the enhanced greenhouse effect from
observations is not likely for a decade or more.”

In 2001, the IPCC Third Assessment
offered a firmer stand: “There is new and stronger evidence that
most of the warming observed over the last 50 years is attributable to
human activities.”

At the behest of President
George W. Bush, the National Academy of Sciences studied global-climate
research and in 2001 issued a report that said: “Greenhouse gases
are accumulating in the Earth’s atmosphere as a result of human
activities, causing surface air temperatures and subsurface ocean temperatures
to rise. . . . Human-induced warming and associated sea level rises are
expected to continue through the 21st century.”

Meanwhile, the two
most influential science journals in the English-speaking world, Science
and Nature, have published dozens of studies detailing the causes
and impacts of climate change.

“The scientific
evidence on global warming is now beyond doubt,” Donald Kennedy,
editor-in-chief of the journal Science, wrote in a recent editorial.
“Readers of these pages during the past couple of years have seen
one careful study after another documenting the role of anthropogenic
sources of carbon dioxide and other greenhouse gases in global warming;
describing the impact of past and present climate change on marine and
terrestrial ecosystems; and measuring rates of glacial melting in the
Arctic, the Antarctic, and on the tops of low-latitude mountains.”

The United States, Kennedy
argues, must take action on global climate change. “In this case,
the stakes are well beyond national interest, because the nonparticipation
of the United States in the global effort on climate change is more than
a national embarrassment. It’s dangerous.”

To some skeptics,
however, the editors of Science and Nature have been
blinded by political considerations. These journals “are agenda-driven
magazines,” says John R. Christy, the Alabama state climatologist
and an IPCC report lead author. “They have decided that climate
change is a threat; therefore it must be dealt with by centrally planned
solutions, taking access to energy away from people, passing regulations
from a government mandate, saying you must reduce your energy consumption.”

Compounding
problems

Climate change is not today’s
most important environmental crisis, some say. The potential consequences
of global warming, they argue, are too far in the future to worry about.
Instead, society should concentrate on overpopulation, suburban sprawl,
excess nutrient pollution, invasive species, and water overexploitation.

The reality, however, is that
global warming already worsens many challenging environmental problems.
“It’s the combination of population, resource use, and climate
change that really makes the nasty brew,” says James White, a climate
scientist at the University of Colorado.

Rapidly growing populations
and agriculture are using vast amounts of water, driving down reservoir
levels and aquifers in some regions, even during times of average local
rainfall. Society therefore often creates “human-demand droughts”
in absence of meteorological droughts.

Starting in the late
1990s, an extremely intense warming in the western Pacific associated
with the La Nina phenomenon pumped additional heat into the atmosphere
and altered winds of the jet stream, changing precipitation patterns in
a band stretching nearly around the world, including much of North America,
from 1998 to 2002, reports Martin Hoerling of NOAA’s Climate Diagnostics
Center and Arun Kumar of NOAA’s Climate Prediction Center in a recent
Science journal article. The U.S. Southeast was one of the areas
hardest hit by this drought.

The hot-temperature spike in
the western Pacific was partly due to global warming consistent with increasing
greenhouse gases, the researchers noted in their article.

Some meteorological
droughts in the United States and elsewhere may have man’s thumbprints
all over them. “That human-contributed greenhouse gases could be
causing widespread, persistent, and very damaging droughts raises the
stakes considerably,” says White.

Sea
levels

Could climate change interact
with a natural cycle in sea level to damage coastal resources?

During the past century, sea
levels have risen by an average of one foot on Gulf and Atlantic beaches
primarily because of seawater’s thermal expansion. When the global
surface temperature becomes hotter, seawater expands, and sea levels rise.

Shorelines have migrated landward
at an average rate of one or two feet per year over the past century,
though local conditions vary greatly. In some areas, the annual erosion
rate is more than 20 feet annually. Other areas are stable or growing.

Sea levels are expected
to rise by an average of 19 inches worldwide between 1990 and 2100, affecting
low-lying countries and some coastal cities and habitats, according to
the World Water Council, an international water agency.

Sea level also naturally
fluctuates during cycles on the order of every 20 years, says James T.
Morris, a marine biologist at the University of South Carolina. For a
single 20-year period, sea level rises overall, though there are ups and
downs within the larger cycle. Then for the next 20-year cycle, sea level
falls overall, though with smaller ups and downs. “The 20-year cycles
are regular and very predictable, but they are not caused by any astronomical
force that we know of, such as the motion of the planets, moon, and sun.”
These cycles remain a mystery, says Morris.

Today, sea levels have neared
the very top of a natural cycle. “Sea level has risen very rapidly
over the last 10 to 15 years, and we are now due for a decline in sea
level over the next decade, provided past cycles repeat themselves. The
fact that we do not understand their cause means that we cannot forecast
with confidence.”

How much of recent sea level
rise is driven by the 20-year cycle and how much is driven by global climate
change? That’s impossible to say.

The ascending side
of the 20-year cycle could end soon. Sea level is likely to fall over
the next two decades, reducing global warming’s impacts on coastlines.
But in the future, an ascending swing of the 20-year cycle could combine
with thermal expansion of seawater to swamp coastal habitats around the
world.

Intergovernmental
Panel on Climate Change (IPCC). Climate Change 2001: Synthesis Report.
A Contribution of Working Groups I, II, and III to the Third Assessment
Report of the IPCC. New York: Cambridge University Press.